1. Field of the Invention
The present invention relates to magnetoresistive sensors for reading magnetically-recorded information from data storage media, and particularly to giant magnetoresistive (GMR) read sensors for direct access storage device (DASD) systems.
2. Description of the Prior Art
By way of background, GMR sensors, also known as “spin valve” sensors, are commonly incorporated in read heads for magnetic media-based DASD systems, such as disk drives. A spin valve sensor is a magneto-electrical device that produces a variable voltage output in response to magnetic field fluctuations on an adjacent magnetic storage medium. As illustrated in FIG. 1, a conventional spin valve device is formed by first and second ferromagnetic layers, hereinafter referred to as a “pinned” layer and a “free” layer, separated by an electrically conductive spacer layer. In a disk drive, these layers are oriented so that one edge of the layer stack faces an adjacent disk surface, in a cross-track direction, and so that the layer planes of the stack are perpendicular to the disk surface. The magnetic moment (M1) of the pinned layer is oriented at an angle θ1 that is perpendicular to the disk surface (i.e., θ1=90°). It is sometimes referred to as the “transverse” magnetic moment of the sensor. The magnetic moment M1 is substantially pinned so that it will not rotate under the influence of the disk's magnetic domains. Pinning is typically achieved by way of exchange coupling using an adjacent antiferromagnetic pinning layer. The magnetic moment (M2) of the free layer has a zero bias point orientation θ2 that is parallel to the disk surface (i.e., θ2=0°). It is sometimes referred to as the “longitudinal” magnetic moment of the sensor. The magnetic moment M2 is free to rotate in positive and negative directions relative to the zero bias point position when influenced by positive and negative magnetic domains recorded on the disk surface. In a digital recording scheme, the positive and negative magnetic domains correspond to digital “1s” and “0s.” The zero bias point is the position of the free layer magnetic moment M2 when the sensor is in a quiescent state and no external magnetic fields are present.
Electrical leads are positioned to make electrical contact with the pinned, free and spacer layers. In a CIP (Current-In-Plane) spin valve sensor, as shown in FIG. 1, the leads are arranged so that electrical current passes through the sensor stack in a cross-track direction parallel to the layer planes of the stack. When a sense current is applied by the leads, a readback signal is generated in the drive processing circuitry which is a function of the resistance changes that result when the free layer magnetic moment M2 rotates relative to the pinned layer magnetic moment M1 under the influence of the recorded magnetic domains. These resistance changes are due to increases/decreases in the spin-dependent scattering of electrons at the interfaces of the spacer layer and the free and pinned layers as the free layer's magnetic moment M2 rotates relative to the magnetic moment M1 of the pinned layer. Resistance is lowest when the free and pinned layer magnetic moments are parallel to each other (i.e., θ2=90°) and highest when the magnetic moments are antiparallel (i.e., θ2=90°). The applicable relationship is as follows:ΔR∝cos(θ1−θ2)∝sin θ2.
The ΔR resistance changes cause potential differences that are processed as read signals. As can be seen from the foregoing relationship, it is important that the magnetic moment of the free layer be directed substantially parallel to the disk surface (i.e., θ2=0°) when the sensor is in its quiescent state. The parallel position corresponds to a zero bias point on a transfer curve of the sensor that represents GMR effect ΔR/R (ratio of change in resistance to resistance of the sensor) as a function of applied magnetic fields. This allows for read signal symmetry upon the occurrence of the positive and negative magnetic field incursions from the recorded magnetic domains on the disk surface. Unfortunately, during the quiescent state there are often magnetic forces acting on the free layer that cause its magnetic moment M2 to rotate from the desired orientation parallel to the ABS (i.e., θ2≠0°). This results in read signal asymmetry in which the potential changes of the positive and negative read signals are unequal, thus producing a reduced readback signal. Accordingly, there is an ongoing effort to balance the magnetic forces acting on the free layer in the quiescent state.
One technique used to orient the free layer's magnetic moment during quiescence is to place electrically conductive hard biasing regions underneath the electrical leads in adjacent coplanar relationship with the free layer to help stabilize the free layer magnetic domains in the desired orientation. Conventional hard biasing regions are made of ferromagnetic material with relatively high magnetic coercivity (Hc), such as CoCrPt and alloys thereof. A property of these materials is that they are electrically conductive. This is not a problem in conventional CIP spin valve designs wherein the hard biasing regions are located in the electrical pathway between the leads that deliver sense current to the device. However, in other designs the electrical conductivity of conventional hard biasing materials may be detrimental to device operation. Consider, for example, an in-stack hard biasing design in which the hard biasing material is located out of the plane of the free layer so as to occupy its own in-stack layer. Such a design could facilitate reduced track widths that in turn would provide increased data storage density. In-stack hard biasing would also eliminate the complicated fabrication of “contiguous junctions” between the hard biasing regions and the free layer, as is commonly used in conventional hard biasing schemes. Notwithstanding these advantages, the conductive properties of conventional hard biasing materials make an in-stack hard biasing design impractical insofar as the sense current could be shunted through the hard biasing region and away from the free, pinned and spacer layers, so as to thereby reduce the readback signal.
Accordingly, a need exists for a GMR sensor configuration wherein in-stack hard biasing is made possible without shunting sense current away from the electrical pathways of the device. What is required in particular is a GMR sensor having an in-stack hard biasing region that is magnetically hard yet electrically insulative.